Communication is the act of transmitting information from one place to another. Every communication system, whether a simple telephone call or a satellite TV link, can be reduced to three essential blocks: a transmitter, a communication channel and a receiver. The information originates at a source and is delivered to a user (destination).
- Transmitter: processes the message signal so it becomes suitable for transmission over the channel and for radiation by the antenna. It typically contains a transducer, a modulator and an amplifier.
- Channel: the physical medium that carries the signal from transmitter to receiver — a pair of wires, a coaxial cable, an optical fibre, or free space (for radio waves).
- Receiver: extracts the original message signal from the received (often weak and distorted) signal through amplification, demodulation and detection.
Transducer: a device that converts one form of energy into another. A microphone converts sound (pressure variations) into an electrical signal; a loudspeaker does the reverse. Communication electronics works only with electrical signals, so transducers sit at the input of the transmitter and the output of the receiver.
Analog vs digital signals. An analog signal is continuous in both time and amplitude — speech and music waveforms are analog. A digital signal takes only discrete values (usually two levels, $0$ and $1$). Digital communication is more robust against noise, can be regenerated cleanly by repeaters, and is easily encrypted, which is why modern systems are overwhelmingly digital.
Signal degradation in the channel. Three effects act on a signal as it travels:
- Attenuation: the loss of signal strength (power) as it propagates, caused by absorption and spreading in the medium. It is measured in decibels (dB).
- Amplification: increasing the amplitude (and power) of a signal using an electronic amplifier. Repeaters placed along the channel pick up, amplify and retransmit the signal to compensate for attenuation.
- Noise: unwanted, random electrical signals that get added to the message during transmission or processing. Noise sets the ultimate limit on how weak a usable signal can be.
Gain in decibels. Because power levels span huge ranges, gain and loss are expressed logarithmically: $\text{Gain (dB)} = 10\log_{10}\dfrac{P_o}{P_i}$, where $P_o$ and $P_i$ are output and input powers. A gain of $+20\,\text{dB}$ means the power increased $100$ times; an attenuation of $-30\,\text{dB}$ means the power fell to one-thousandth.
Bandwidth of a signal. Every message occupies a band of frequencies. The bandwidth is the range $\Delta f = f_{\max} - f_{\min}$ that must be transmitted faithfully. Typical message bandwidths are: speech for telephony about $2800\,\text{Hz}$ ($300$ Hz to $3100$ Hz); high-quality music up to $20\,\text{kHz}$ (audible range $20\,\text{Hz}$ to $20\,\text{kHz}$); a video signal about $4.2\,\text{MHz}$; and a composite TV signal (video + audio) about $6\,\text{MHz}$.
Bandwidth of transmission media. The carrying capacity of a medium increases with its usable frequency range. Twisted-pair telephone wire handles a few hundred kHz; coaxial cable supports up to about $750\,\text{MHz}$; and an optical fibre, working at optical frequencies near $10^{14}\,\text{Hz}$, offers an enormous bandwidth (tens of GHz to THz), which is why fibre carries the bulk of the world's high-speed data with very low attenuation.